you have the powered by USB jumper removed and the 24v power hooked up? If so I would remove all other cables you have plugged in to see if something is causing the problem. I pinched a USB cable in the past to a camera that was causing issues on my USB bus and cause similar issues. Take it down to the basics, power to the pi, 24v power to the octopus and a USB cable between the two and thats it. See if that works and if so work up from there